The present invention relates generally to input means for apparatus of the type having a digital controller and more particularly to a key input device so designed that the identification of a depressed key is determined by the combined use of an analog-type signal and a digitaltype signal.
The receat development in the field of semiconductors have been so remarkable that the construction of control systems with electronic circuits has advanced rapidly. Thus, the construction of control systems and the shapes of control panels have undergone considerable changes and the construction of their input means have also undergone changeover from mechanical switches mainly to those types which discriminate the respective keys by means of a microcomputer through the medium of electric signals, thereby performing the desired control.
In addition, a wave of digitization has resulted in the addition of new functions one after another and the trend has been toward increasing the number of operating keys for effecting such functions.
However, the increase in the number of such keys has resulted in an increase in the input and output terminals of the microcomputer and hence the number of peripheral circuits has been increased from the construction standpoint of the system.
In view of these circumstances, as shown in FIG. 1A of the accompanying drawings attempts have been made to decrease the number of input and output terminals in proportion to the number of operating keys mainly by use of a key-scan matrix, in which a matrix circuit is formed by an output signal system and an input signal system so as to discriminate the kinds of a respective depressed keys with the reduced number of inputs and outputs.
This type of system includes a plurality of output ports R and a plurality of ordinary input terminals K so that scan pulses of a controlled timing are generated from the output ports and the corresponding pulse is applied to the input terminal connected to the depressed key, thereby discriminating the key.
However, this system is also not a method which drastically decreases the number of the required input and output terminals and therefore it is limited by the number of the input and output terminals of the microcomputer. Thus, if these input and output terminals are used only for operating key inputting purposes, there is a shortage of the input and output terminals for the other essential control purposes. Moreover, it leads to a pressure for the capacity of the ROM of the microcomputer used with this system for scan output control purposes, etc.
Also, as another example of the conventional methods, there has been a system which utilizes analog inputs. As shown in FIG. 1B, this system is of an analog-to-digital conversion (A/D conversion) type in which voltages generated by the depression of the respective keys differ from one another and each key is discriminated by converting the corresponding analog voltage to digital form.
Since this system includes an A/D conversion input terminal and each key is discriminated in accordance with a variation in the voltage generated by its depression, when the key at the fifth row from above in FIG. 1B is depressed, the following voltage is generated ##EQU1##
However, this system is also disadvantageous in that an increase in the number of keys requires a greater resolution for the A/D conversion and therefore it is necessary to use high-precision resistors for generating a voltage corresponding to each key, thus making the system unfavorable from the standpoint of cost and working performance.